This invention relates to electrical resistance heater controllers and particularly to such controllers for heaters embedded in aircraft windshields that are to be fog and ice free.
Aircraft windows are normally a laminated assembly of glass or plastic of which one of the lamina is electrically conductive and serves as an electric heating element to avoid condensation and ice buildup. The window heating element is supplied AC current, typically at 400 Hz from the usual aircraft generators. A window heat controller is used with the heating element to hold the windshield temperature within an appropriate selected range. Window heat controllers are intended to provide efficient use of electrical energy, low cost and weight, good operational reliability, and easy maintenance.
Various known window heat controllers include the following:
On-off control--This form of controller applies full current to the window until its temperature reaches a predetermined value at which time the current is reduced to zero and is reapplied at full value when the temperature drops to a lower predetermined value. In performance, such control is adequate for relatively small heating loads, such as about 200 to 1000 watts, but results in unacceptable power supply line disturbances for larger heating loads ranging up to 3000 or 4000 watts.
Tap changer control--In this form of control a key element is a transformer with several taps so that the value of current applied to the window can be varied by selection of one of the different taps. Thus, when the window temperature reaches an upper predetermined value, the next lower tap is selected and the tap changing process continues to apply more or less current to the window to maintain its temperature within a narrow range. This form of control is susceptible to an undesirable failure mode when implemented with solid state switches. If one tap switch fails shorted, a tap-to-tap short occurs when the adjacent tap switch is closed. This is a possible source of overload on the transformer that could lead to overheating and fire.
Phase angle control--In this type of control scheme, the conduction period of each half cycle of current is varied to meet window heat requirements. Thus, for maximum heat, heating current would flow for the full 180.degree. of each half cycle. For application of half power, no current would flow from, say, 0.degree. to 90.degree., then current would rise to its peak value and decrease to 0.degree. at 180.degree. to obtain the necessary heating. This has the disadvantage of requiring heavy filtering to buffer the supply line from the step changes in current when other than zero or 180.degree. conduction occur. Various schemes employing magnetic amplifiers whose output power level is proportional to sensor demand are known which operate in principle similarly to that of the phase angle control apparatus.
Pulse modulated controller--The pulse modulated controller is an energy proportional design that requires one pair of solid state switches per phase to conduct current to the load. As used it is intended to switch the entire AC cycle current to the load with the number of controller output cycles made proportional to the temperature sensor demand. Such apparatus may cause amplitude modulation to the power source or phase voltage unbalance in single phase or two phase designs.